The following code fails with an exception even though the service states exportTilesAllowed=true.
Exception: "Tile export is not enabled: Vector tile service does not support exporting tiles"
I've included code for loading the basemap using ArcGISVectorTiledLayer which accurately parses the exportTilesAllowed as true. See the screenshot.
var uri = new Uri("https://services.geodataonline.no/arcgis/rest/services/GeocacheVector/GeocacheBasisTerreng/VectorTileServer");
var basemap = new ArcGISVectorTiledLayer(uri);
await basemap.LoadAsync();
try
{
var exportVectorTilesTask = await ExportVectorTilesTask.CreateAsync(uri);
}
catch (Exception e)
{
// fails with: "Tile export is not enabled: Vector tile service does not support exporting tiles"
throw;
}

I've also tested with "https://basemaps.arcgis.com/arcgis/rest/services/World_Basemap_Export_v2/VectorTileServer" which works just fine.
Using ArcGIS Runtime for .NET v100.13 @WinUI 3